US 6429637 B1 Abstract An electronic power meter for metering the consumption of electrical energy on power lines includes phase compensation on current transformers, whereby the acquisition of one of the two samples for the current signals is delayed and time shifted averaged, such that the average of the two signals provides a compensated signal. The amount of delay is determined from the phase lag the current transformers exhibit during the process of calibration for phase compensation. The amount of compensation that is applied varies with current, thus compensation for the non-linearity in the phase shift for the current transformers. The degree of non-linearity is computed which results in only two variables that define the phase lag at higher current and the degree of non-linearity. The technique helps in using inexpensive current transformers in the meter.
Claims(8) 1. An electronic power meter for metering the consumption of electrical energy in polyphase power lines, comprising:
at least one current channel defined by a passive current transformer with a primary and secondary winding having line current signals associated therewith;
at least one voltage channel associated with a respective current channel and having phase voltage signals associated therewith;
a processor with a sequential sampling analog to digital converter for digitizing phase voltage and line current signals on associated current and voltage channels, wherein
said processor initially samples said current channels for a particular sample and following a selected time delay samples the associated voltage channel, said processor samples said current channel a second time following the sampling of said voltage channel for said particular sample, and stores the resulting digitized signals for use in performing a weighted average, the amount of time delay and the weights for averaging is determined on the basis of amount of phase lag that is to be compensated.
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Description The invention relates to an electronic power meter using a processor for all its computations and includes compensation of phase lag on the current transformers used for sensing the line currents. Conventional power meters utilize current transformers for sensing line currents. As inductive elements, all current transformers exhibit definite phase lag between the primary and the secondary. The error due to this phase lag affects the accuracy of measurements and thereby resulting in overall inaccuracy. Power meter manufacturers are used to implementing conventional phase shifters using variable resistors and/or variable capacitors for each of the three phases increasing the material and production cost. Moreover, variable RC network phase shifting is not applicable to multiplexed signals in three phase electronic power meters. Phase compensation techniques have been previously described, for example, in U.S. Pat. Nos. 5,017,860 and 5,231,347. Techniques have also been developed that use two analog-to-digital converters and by shifting the sampling time of one of the external circuitry increasing the cost of the hardware. These techniques do not compensate for the non-linearity in the phase shift with respect to the current that exists on the current transformers. It is therefore an object of the invention to provide a simplified phase adjustment for an electronic power meter. It is another object of the invention to provide the exact amount of phased shift required for each of the current transformers on each of the phases, which could be different. It is yet another object of the invention to provide different amount of phase compensation at different currents to compensate for the non-linearity on the current transformer. In practicing the invention, an electronic solid state power meter is provided for single phase power line and polyphase power lines utilizing a multi-channel analog to digital converter in built in the processor, to first provide a phase compensation on the current and voltage signals using time shifted averaging technique implemented in software and then subsequently using this compensated data for computing the powers and energies. The invention is very useful and is not computational intensive technique. In accordance with the invention, a processor compensates the phase shift on current transformers by first estimating the delay between two samples of current signal for subsequent averaging in computations in software. Estimating the amount of desired variation of delay with respect to current will not only compensate for the phase shift on the current transformers, but also compensates for the non-linearity in the phase shift. The phase voltages are scaled down to lower voltages using potential dividers and currents are fed to the primary of the current transformers. The secondary of the current transformer provides a current output proportional to turn ratio between the secondary and the primary of the transformer. A resistor with very low temperature coefficient terminated on the secondary of the current transformer provides a potential proportional to the value of the resistance and the current in the secondary. These signal conditioned phase voltage and line current signals are fed to the analog to digital converter, which is part of the processor, to digitize at periodic intervals. The digitized signal are used by the processor to multiply and compute instantaneous powers and are then integrated for a finite number of mains cycles to compute energy. The computed energy value is stored in the internal non-volatile memory at desired intervals of time. FIG. 1 is a schematic block diagram of a three-phase electronic power meter in accordance with the invention; FIG. 2 is a schematic block diagram of a single-phase electronic power meter in accordance with the invention; FIG. 3 is a plot of in-phase current and voltage signals; FIG. 4 is a plot of current and voltage signals illustrating a delayed current signal with respect to the voltage and delay caused by the signal lag on the secondary of the current transformer with respect to the primary; and FIG. 5 is a plot of phase lag with respect to current. The invention has been implemented in an exemplary three-phase electronic power (watt-hour) meter FIG. 1 is a schematic block diagram of an exemplary three phase electronic power meter If the there is no phase lag between the primary and the secondary of the current transformer (an ideal situation), then the time between the current sample a and the voltage sample v, and the time between this voltage sample v and the current sample b is equidistant or the same. Then the average of the current sample a and current sample b will fall exactly on the voltage sample. If the current waveform is lagging behind the voltage waveform by, for example, 0.1 degrees (in case of a 50Hz sine wave (mains)), it will take 20 milli-seconds for one full wave, which could be equivalent to approximately 5.6 micro-seconds lag. Once the first sample is preponed by, for example, 11.2 micro-seconds, then the average of the two current samples will be a compensated current value for the CT. In an ideal situation, when there is no phase shift on the secondary of the current transformer with respect to the primary, the current signals on lines The scaling factor on the phase voltages by the potential dividers The digitized signals from the analog to digital converter As an optimum method of calibration for the phase shift on the current transformers the current is applied at Due to the inherent phase lag on the current transformers LCD FIG. 2 is a schematic block diagram of an exemplary single phase electronic power meter The acquisition of current and voltage signals and further processing of current signal is exactly same as in the case of the three-phase electronic power meter shown in FIG. Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. 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